Central to understanding the mechanisms that produce postures and movements is the determination of the principles by which the smallest functional units of the motor system, motor units, are combined into activity. Yet to be determined are the rules that regulate and coordinate the activities of motor units that are recruited together as an ensemble from muscles throughout a limb or body segment. The long term goal of this proposed project is to make that determination for animals with normal muscles, as well as for animals recovered from nerve injury. To achieve this goal, the project brings together methodologies for making detailed measurements of single motor units with those for obtaining global assessments of limb movement. Using decerebrate cats, the axons of motoneurons supplying different hindlimb muscles will be isolated by micropipettes for purposes of recording and stimulation. The action potential activity and other attributes of the units will be studied together with patterns of whole- muscle activity and ankle and knee joint torques. All measurements will be made in the context of motor actions elicited by stimulating different peripheral and central afferent inputs to motoneurons. Two experimental groups will be examined, one with untreated nerves and one recovered from the section of nerves supplying a synergistic group of muscles. The results of these experiments will meet three specific aims. First, the composition of motor-unit ensembles will be determined in order to identify principles underlying the basis for selection of motor units in particular motor operations. Second, the order of recruitment will be determined for units in the ensemble in order to test the extent to which these units are ordered by size and other functional attributes. Third, ensemble formation and recruitment order will be tested in muscles that have regained the capacity for force generation one year after nerve section. This third aim will provide needed information about the reason for limited motor recovery from nerve injury and allow critical examination of the role played by muscle proprioceptor in organizing motor-unit activity.